1. Field of the Invention
The invention relates to a lead frame on which a semiconductor chip is mounted.
The invention also relates to a semiconductor device which has a semiconductor chip mounted on the lead frame and sealed with a resin, and more particularly to a surface mounting type semiconductor device.
Besides, the invention relates to a manufacturing method of a semiconductor device.
2. Description of the Related Art
Electronic products having semiconductors are extensively used in various fields, and semiconductor chips being developed have higher intelligence and integration. Semiconductor devices such as a semiconductor package, which has a semiconductor chip mounted on a lead frame and sealed with a resin, are also demanded to have higher reliability and productivity.
The lead frame for semiconductor devices is generally made of an Fe--Ni based alloy such as 42 wt % Fe--Ni, and 29 wt % Ni-17 wt % Co--Fe. Such an Fe based lead frame has a disadvantage that production costs become increasing because expensive Ni, Co and the like are used as the alloy ingredients.
The lead frame is demanded to have high heat radiation, namely high heat conductivity as the semiconductor chip is highly integrated. However, the Fe based lead frame also has a disadvantage that it cannot fulfill such a demand.
Under such circumstances, a copper based alloy, which is relatively inexpensive and has excellent heat conductivity and electric conductivity, is being used for the lead frame. Materials for such a lead frame include, for example, Cu--Sn based, Cu--Sn--Ni--Si based, Cu--Fe--P based, Cu--Sn--Ni--Si--Zn based and Cu--Fe--P based alloys.
However, where a resin-sealed semiconductor device is configured to have a lead frame made of copper or copper alloy, it has a disadvantage that intimate contact and adhesion strength between the lead frame and the sealing resin are insufficient.
If the intimate contact between the lead frame and the sealing resin is insufficient, moisture resistance, weathering resistance and corrosion resistance of the semiconductor chip in the semiconductor device are adversely affected, and the reliability of the semiconductor device is heavily degraded.
Especially, a resin-sealed surface mounting type semiconductor device has a disadvantage that the sealing resin is cracked due to a thermal stress when it is mounted on a circuit board. Therefore, the reliability of the semiconductor device is heavily degraded and its productivity is also decreased.
FIG. 18 and FIG. 19 are sectional views schematically showing an example of the configuration of a conventional semiconductor device.
This semiconductor device has a semiconductor chip 92 mounted on a lead frame 91 with an adhesive layer 93 between them and sealed with a sealing resin 94. The semiconductor chip 92 has a first face and a second face, which is fixed to face the first face with a die pad 91d of the lead frame, on which the semiconductor chip is mounted, with the adhesive layer 93 between them. The semiconductor chip 92 has connecting terminals 92b on the second face, and, these connecting terminals 92b are electrically connected with inner leads 91e of the lead frame by bonding wires 95 respectively.
Occurrence of cracking will be described briefly.
As described above, the semiconductor device has an interface, which is low in intimate contact and easy to remove or separate, such as the interface between the sealing resin 94 and the lead frame 91 in it. On the other hand, the sealing resin 94 configuring the semiconductor device absorbs moisture contained in the atmosphere and the like. If the semiconductor device is exposed to a high temperature when it is mounted, the moisture absorbed by the sealing resin 94 is evaporated. If an internal stress due to vapor is applied to a separated face in the semiconductor device, the semiconductor device is swelled. Further, if the internal stress exceeds a breaking strength of the sealing member, cracks 99 occur in the package (FIG. 19).
This occurrence of cracks in the semiconductor device is a serious problem in connection with enlargement of the semiconductor chip and thinning of the semiconductor device in these years.
Therefore, so-called moisture-proof packing, that the semiconductor device is placed in an aluminum-coated bag for shipping, is adopted today, and use time of each product after opening the bag, namely time of absorbing moisture, is controlled to prevent cracks from occurring in the semiconductor device. Thus, a large burden is on both the semiconductor manufacturers and users, decreasing productivity.
The semiconductor device having the copper based lead frame also has a disadvantage in its forming property, which is conspicuous in connection with the enlarging and thinning of the semiconductor device.
The semiconductor device having a copper lead frame has a thermal expansion coefficient different by about one digit from that of silicon which is a main member for the semiconductor chip. Therefore, when they are cooled down to room temperature after adhering mutually in the die attach process, a warp is caused by a thermal stress. A displacement magnitude becomes larger due to the warp as the semiconductor chip becomes larger in size, causing a serious problem.
A large semiconductor chip generally has multiple points for connection, and its corresponding lead frame has a fine pitch, especially very fine for the inner leads. To keep the proper mutual positional relationship of the very fine and many inner leads of the lead frame, they are generally fixed by a tape which is made of a resin such as polyimide.
However, there is a disadvantage that the tape for keeping the inner leads in the proper position shrinks due to a thermal load applied in the assembling process, causing the lead frame warping. The warp makes it further difficult to form a large and thin semiconductor device which is generally hard to produce, causing failures such as a die shift and exposure of the semiconductor chip or the die pad.
Improvement of the forming property needs the sealing resin formed to have substantially the same thickness on the upper side of the semiconductor chip and the lower side of the die pad. Therefore, the lead frame is depressed. If the warp problem cannot be remedied, a depressing amount must be determined taking a warping amount into account. But, it is not easy to estimate an accurate warping amount, and its optimum value is determined by trial and error. Such a method is hard to produce a highly reliable semiconductor device at high productivity.
The present invention was completed to remedy the disadvantages described above. It is an object of the invention to provide a semiconductor device having a lead frame which is intimately contacted to a sealing resin.
It is also an object of the invention to provide a semiconductor device, which has good intimate contact between a sealing resin and a lead frame and also high reliability and productivity.
It is also an object of the invention to provide a semiconductor device having high forming property. It is another object of the invention to provide a method of producing a semiconductor device which has good intimate contact between a sealing resin and a lead frame and also high reliability and productivity.
It is still another object of the invention to provide a method of manufacturing a semiconductor device, which has good intimate contact between a semiconductor chip and a sealing resin and also high reliability and productivity.
Besides, it is an object of the invention to provide manufacturing equipment of a semiconductor device having good intimate contact of a sealing resin and high productivity.